Follow path constraint deforming mesh
enabling Follow curve option  in the follow path constraint force the mesh to deform according to the path "like a curve Modifier does" :



to follow the curve without a distortion :


add an empty with a follow path constraint on the same path 
adjust the offset of the follow curve of the object  " 1.00 or according to your path"
add a locked track constraint to the object and set the target to the Empty object
keep adjusting the offset till you get a nice result